I have posted this question on eng-tips too.
I found out that a lot of people are getting confused with this question and I read a lot of interesting discussions but got to no conclusion. Any help on this will be really appreciated.
I have a situation in which I have a column channel which is welded (all around) to a base plate. The base plate is bolted to the concrete slab by 4 bolts. For these type of situations, I used to consider the base as pinned and design base plate for shear/axial loads only. I know that’s not totally true as there is always some rotation restraint but that was a conservative assumption.
Recently my supervisor considered the same situation as fixed and I am really confused as to why he did that. He actually went ahead and modified the base conditions further. He considered base to be pinned in minor axis bending of the channel and fixed for major axis bending. Can someone explain to me as to how these conditions are fixed and pinned in another direction?
Welcome person… I was going to use guy, but, to be gender correct, I changed my mind.
Unless I’m dealing with a frame or portal, I almost never consider column bases as fixed. and do not design the foundation for any moment this ‘pinned’ connection will transfer… significant eccentricities only.
I don’t know why he would treat the axis any differently unless you have a very flexible base plate. A bit of a puzzle. You might want to ask him his reason for this.
By virtue of the axial load, there is some degree of fixity about both axis; this I ignore.
Columns are one of the few structural items that I tend to be conservative with design; they generally don’t redistribute loads very well.
You can conservatively treat a base plate as pinned. So if your supervisor wants to treat the minor axis as pinned, that’s fine. Most base plates are partially fixed but we treat them as pinned. It’s like treating clip angle (simple) steel connections as pinned even though there’s some fixity. It’s hard for a channel in the minor axis to be fixed because of the low moment of inertia in the minor axis, so you can consider it pinned.
If you have to treat the major axis as fixed, I would follow the procedures in the AISC Design Guide 1 (base plate and anchor rod design). The anchor rods have to be outside the flanges for it to be fixed. There’s a different calculation for small moments and large moments. There’s tension in some anchor rods.
It might help to think about a steel moment connection. You get tension and compression in each flange. Same thing with a base plate moment connection.
edit: Although base plates might be flexible and they might not make a good fixed connection, there is a published AISC guide on the subject, so I think it’s okay to use it.
Welcome to SE. Looking at the sketch you posted on ET, I would say, if anything is fixed, it would be the minor axis. There is no real fixity, however and I agree with taking the column as hinged in both axes.
I agree with the assumption to use Pinned connections for column bases. It is conservative but typically yeilds less expensive foundations. To reduce the size of your columns, it can be considered fixed so long as you baseplate, anchor bolts and foundations can take the applied moment (Bolts should always be outside of the column, not inside flanges).
However, when doing seismic design, special attention must be paid. If you define a SFRS (Shear Force resisting system), there are rules which must be followed and columns are typically pinned in these cases.
Mostly it comes with experience, so I agree with @milkshakelake that you should ask your supervisor about his logic behind it so you can add it to your knowledge base.